Lg monitor suddenly not detected by mac M1 pro

Question

Chantus Author

Level 1

15 points

Lg monitor suddenly not detected by mac M1 pro

Hello everyone,

I have a persistent issue with my external monitor that I haven’t been able to solve.

Mac: MacBook Pro with M1 Pro chip
macOS: Sequoia 15.6.1 (problem was also present on Sequoia 15.0 before updating)
Monitor: LG 27UL500 (4K)
Cable/Connection: HDMI (direct and via USB-C hub)

Problem

My MacBook no longer detects the LG 27UL500 at all.

The screen turns on (black screen, no “no signal” message), but the Mac does not list it under Displays in System Settings or in About This Mac > System Report > Graphics/Displays.
Only the built-in display is shown.
This monitor worked perfectly with this Mac and this same HDMI cable for two years without any issue.

Tests already performed

Cable check: The HDMI cable works fine with another device (Atomos Shinobi monitor).
Monitor check: The LG monitor works fine when connected to another source (DJI RC Pro controller and another MacBook Pro Intel).
Mac check: The same issue happens with direct HDMI adapter and through a USB-C hub.
Force detect: Tried “Detect Displays” in Display Settings (holding Option key). No detection.
PRAM/NVRAM reset: On Apple Silicon the reset happens automatically on boot, but I also tried clearing NVRAM via Terminal in Recovery (nvram -c). No change.
SMC reset equivalent: Full shutdown and power-on after 30 seconds. No change.
Safe Mode: Booted into Safe Mode and reconnected the monitor. Still not detected.
New user account: Created a fresh macOS user account. Issue persists.
macOS update: Updated from 15.0 to 15.6.1 hoping it would solve it. Still the same.
Different adapters: Tested both direct USB-C → HDMI adapter and a hub with HDMI output. Same result.
Comparison: The same monitor + same cable works instantly with another MacBook (Intel).

Conclusion

The problem is clearly on my MacBook Pro M1 Pro, not the cable or the monitor.
It seems to be a handshake/EDID issue where macOS does not detect the external display at all.

Question

Has anyone experienced a similar issue with the LG 27UL500 (or other LG 4K monitors) on Apple Silicon Macs after macOS Sequoia updates?

Are there any known workarounds (forcing EDID, switching to DisplayPort, etc.)?

Any help would be greatly appreciated.

MacBook Pro 16″, macOS 15.6

Posted on Aug 27, 2025 04:42 PM

Reply

Answer 1

Aug 27, 2025 05:28 PM in response to Chantus

that LG 27UL500 display appears to be a 4K display with up to 10 bits/color

Interfaces include:

2x HDMI 2.0

1x DisplayPort 1.4

to connect to the M1 Mac you have at that resolution with HDR=ON, you would need special HDMI cables:

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

You could also connect with a USB-C to DisplayPort adapter/cable, provided its USB section was no longer that ONE meter

or you could try turning HDR OFF.

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Answer 2

Chantus Author

Level 1

15 points

Aug 28, 2025 03:11 PM in response to Grant Bennet-Alder

Well, the cable is the original one that came with the monitor and worked great till a few days ago. I've tested the cable (an my mac) with a Xiamoi 32" 4k monitor of my son, and it works fine as well. So the issue seems not to be the cable.

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Answer 3

Aug 28, 2025 04:24 PM in response to Chantus

Cables "shipped in the box", especially HDMI cables, are often 'Lowest bidder' cables, and were thrown into the box at the last minute. They are generally NOT useful at highest resolutions.

unless it was sold as at least a "PREMUIM" cable, or still has the certification tags that indicate it is, then the cable remains suspect.

Newer versions of MacODS have become increasing fussy about PERFECT data transmission. if it makes an error, I gets shut off.

if the connection almost works, it will work in some case and not others, for no obvious reasons.

That's why I recommend you get a known good high quality cable to rule that out.

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Answer 4

Chantus Author

Level 1

15 points

Aug 29, 2025 03:34 PM in response to Grant Bennet-Alder

Solution: Restoring a clean windowserver preference file

After a lot of testing (cables, hubs, reinstalling macOS, NVRAM reset, SwitchResX, etc.), the root cause turned out to be a corrupted display preferences file in my user Library.

Here’s what solved it:

I went into Time Machine and restored an earlier version of

~/Library/Preferences/ByHost/com.apple.windowserver*.plist

After replacing the current file with that backup and restarting, the monitor was immediately recognized again.

It seems macOS was “stuck” with a bad configuration (possibly forcing HDR/YCbCr modes) and refused to negotiate correctly over HDMI. Restoring the older windowserver.plist reset the display configuration and fixed the issue. I've tried just deleting it, but that didn't work.

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Answer 5

Aug 29, 2025 07:15 PM in response to Chantus

For MacOS 12 Monetary and some later versions, some users got relief by deleting some preferences files, then Restarting:

Library/Preferences/com.apple.windowserver

~/Library/Preferences/com.apple.windowserver.<UUID>.plist -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist

-- where [~] tilde is an active abbreviation for "current user"

-- and <UUID> is a string of hexadecimal numbers unique to your system, grouped as 8-4-4-4-8 hexadecimal digits

Restart REQUIRED after these deletions.

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